I. Field of the Invention
The present invention relates generally to echo cancellers. More particularly, the present invention relates to a novel and improved system and method for reducing the chopiness heard in full-duplex systems that use echo cancellers. The teachings of the present invention apply to acoustic echo cancellers, as well as to echo cancellers in networks and other non-acoustic systems.
II. Description of the Related Art
Acoustic echo-cancellers (AEC) are used in teleconferencing and hands-free telephony applications to eliminate acoustic feedback between a loudspeaker and a microphone. In a cellular telephone system where the driver uses a hands-free telephone, acoustic echo cancellers are used in the mobile station to provide full-duplex communications. A block diagram of a traditional acoustic echo canceller is illustrated in FIG. 1.
For reference purposes, the driver is the near-end talker with input speech signal v(n) and the person at the other end of the connection is the far-end talker with input digital speech signal x(n). The speech of the far-end talker is broadcast out of loudspeaker 2 in the mobile telephone. If this speech is picked up by microphone 10, the far-end talker hears an annoying echo of his or her own voice. The output of microphone 10, r(n), is a digital signal. Typically the functions performed by microphone 10 may be accomplished by a microphone, which would convert the audio signal to an analog electrical signal and an analog to digital (A/D) converter. The AEC identifies the impulse response between speaker 2 and microphone 10, generates a replica of the echo using adaptive filter 14, and subtracts it in summer 12 from the microphone output, r(n), to cancel the far-end talker echo y(n). Since the adaptive filter cannot generally remove all of the echo, some form of echo suppression provided by residual echo suppression element 18 (e.g., a non-linear post processor) is typically employed to remove any residual echo.
In FIG. 1, the far end talker echo signal y(n) is illustrated as the output of an acoustic echo path element 4, which is an artifact of the proximity of the loud speaker 2 and microphone 10. To the far end talker echo signal y(n) is added noise signal w(n) and near-end speech signal v(n), illustrated by summing elements 6 and 8 respectively. It should be noted that summing elements 6 and 8 an d acoustic echo path 4 are artifacts of the mobile environment and are presented for illustrative purposes.
Adaptive filter 14 uses the far-end speech x(n) as a reference signal. If adaptive filter 14 is allowed to adapt in the presence of v(n), the near-end speech will be added to the error signal e(n), which drive s the filter tap coefficient adaptation, corrupting the estimate of acoustic echo path 4. It is therefore necessary to disable coefficient adaptation when both talkers are speaking, a condition referred to as doubletalk. During doubletalk, residual echo suppression element 18 must also be disabled to prevent corruption of the near-end speech. A doubletalk detector (not shown) typically detects the presence of doubletalk and provides control signals to disable adaptive filter 14 and residual echo suppression element 18 when doubletalk is present.
As shown in FIG. 2, in prior art echo cancellation systems, residual echo supression element 18 functions to cancel any residual echo by muting (i.e., gating-off) its output whenever an echo is detected by the adaptive filter 14. This aspect of element 18 is shown in portion xe2x80x9cAxe2x80x9d of FIG. 2. The upper half of portion xe2x80x9cAxe2x80x9d of FIG. 2 illustrates the detection of an echo condition resulting from acoustic feedback between loudspeaker 2 and a microphone 10 when the word xe2x80x9cHELLOxe2x80x9d is spoken by a far-end talker. The lower half of portion xe2x80x9cAxe2x80x9d of FIG. 2 shows that, as soon as the echo is detected at t1, the output of element 18 is muted-off entirely. Thereafter, as soon as the echo ceases to be present at t2, the output of element 18 is unmuted.
In addition, in instances where the output of residual echo supression element 18 is muted because adaptive filter 14 has detected an echo and the double-talk detector simultaneouesly detects double-talk, residual echo supression element 18 will unmute its output during the double-talk period. This aspect of element 18 is shown in portion xe2x80x9cBxe2x80x9d of FIG. 2. The upper half of portion xe2x80x9cBxe2x80x9d of FIG. 2 illustrates the detection of a double-talk condition at t4 at a time when the output of element 18 is being muted as a result of the detection of an echo condition between t3 and t4. The double-talk condition results from simultaneous speech by the far-end and near-end talkers (i.e, the near-end talker is saying xe2x80x9cHIxe2x80x9d during the time that the far-end talker is saying xe2x80x9cHELLOxe2x80x9d). The lower half of portion xe2x80x9cBxe2x80x9d of FIG. 2 shows that, as soon as the double-talk is detected at t4, the output of element 18 is unmuted. Thereafter, as soon as the double-talk condition ceases to be present at t5, the output of element 18 is muted again. The output of element 18 remains fully muted until the echo resulting from the word xe2x80x9cHELLOxe2x80x9d spoken by the far end talker ceases at t6. Aas soon as the echo ceases to be present at t6, the output of element 18 is unmuted.
Portion xe2x80x9cCxe2x80x9d of FIG. 2 similarly illustrates an example where the output of element 18 remains unmuted when a double-talk condition is detected between, t7 and t8. Thereafter, as soon as the double-talk condition ceases to be present at t8, the output of element 18 is muted again because of an ongoing echo condition. The output of element 18 remains fully muted until this echo condition ceases at t9, thereby causing muting of the word xe2x80x9cUPxe2x80x9d from the near-end talker between t8 and t9. As soon as the echo ceases to be present at t9, the output of element 18 is unmuted.
Referring still to the prior art system of FIG. 1, when the echo component y(n) dominates the near-end speech signal v(n) during particular periods, the near-end speech will be muted out by element 18. This situation can result in an undesireable chopiness in the audible signal heard by the user. One purpose of the present invention is smooth this chopiness, which can be unpleasant and annoying to the user.
In addition to be annoying to the user, this chopiness often results in the presentation of an unsmooth energy curve to the vocoder used for encoding the near-end speech signal. In applications where variable rate vocoders are used, the chopiness in the energy curve can cause the vocoder to transmit at the full data rate more often than is necessary, thereby wasting limited system capacity. Thus, it is a further object of the present invention to present a smoother energy curve to the vocoder, thereby improving its performance.
These problems and deficiencies are recognized and solved by the present invention in the manner described below.
One aspect of the present invention is directed to a system and method for canceling an echo signal. This aspect of the invention may be used, for example, for removing an abrupt transition in the audible signal that would otherwise occur when the near-end speech signal is dominated by an echo and the output of the non-linear post-processor is being changed from a non-muted to a muted state. According to this aspect of the invention, an input waveform is provided to an acoustic processor, and a determination is made whether the input waveform includes information representative of an echo signal. If the input waveform includes information representative of an echo signal, an output waveform is formed by attenuating a residual waveform with the acoustic processor. The residual waveform is attenuated by an attenuation factor that gradually changes from an initial attenuation value to a final attenuation value during the attenuation step. This aspect of the invention may also be used, for example, for removing the chopiness in the audible signal that would otherwise occur when the near-end speech is dominated by an echo and the output of the non-linear post-processor is being changed for a non-muted to a muted state.
In accordance with a further aspect, the present invention is directed to a system and method for adjusting an acoustic signal from a muted state to an unmuted state by varying an attenuation factor applied to an acoustic signal by an acoustic processor. This aspect of the invention may be used, for example, for removing the chopiness in the audible signal that would otherwise occur when the end of an echo condition is detected and the output of the non-linear post-processor is being changed from a muted to a non-muted state. According to this aspect of the invention, an acoustic signal is provided to an acoustic processor, and an output waveform is formed from the acoustic processor by adjusting the attenuation factor from the muted state to a first attenuation value associated with the non-muted state. After the attenuation factor is adjusted to the first attenuation value, the output waveform is formed by gradually changing the attenuation factor from the first attenuation value to a second attenuation value. The input waveform is attenuated by a smaller amount when the second attenuation value is applied to the acoustic signal than when the first attenuation value is applied to the acoustic signal. This aspect of the invention may also be used, for example, for removing the chopiness in the audible signal that might otherwise result when a mobile phone is operating in AMPS mode and bursty noise is present.